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Four to 6 wk after parturition, 12 cows in second, fourth, or fifth lactation were experimentally infected in one gland with Escherichia coli. The capacity of chemotaxis, phagocytosis, oxidative burst, and expression of CD11/CD18 receptors to predict the severity of IMI was measured. Bacterial counts in the infected quarter, expressed as area under the curve, and residual milk production in the uninfected quarters were compared to determine severity of the infection. Although these two outcome parameters were highly negatively correlated, regression models with preinfection tests for leukocyte function fitted best with bacterial counts as an outcome parameter. Of the preinfection tests for leukocyte function, chemotaxis best predicted the outcome of the IMI that had been experimentally induced by E. coli. The number of circulating peripheral leukocytes just prior to inoculation was used to predict 52 and 45% of the severity of IMI for bacterial counts and residual milk production, respectively. As a categorical variable, parity predicted 75 and 56% of the severity of IMI expressed as bacterial counts and residual milk production, respectively. Because of the strong effect of parity on the outcome of the experimentally induced mastitis, analysis was performed to discriminate between second parity cows and older cows. Significant differences were found for the number of circulating peripheral leukocytes and for the expression of CD11b/CD18 and CD11c/CD18 receptors between younger and older cows.

The relationship between the severity of experimental Escherichia coli mastitis and the chemotactic response of blood polymorphonuclear leukocytes was investigated before and during mastitis. Experimental E. coli mastitis was induced in 10 healthy cows by inoculation of the rear right quarters with 10(3) cfu of E. coli. Cows were classified into two groups based on the severity of the mastitis. Bacterial growth in the inoculated quarter was used as parameter that indicated severity. Before and during experimental mastitis, the chemotactic response and the number of circulating polymorphonuclear leukocytes were greater for the moderately diseased cows than for the severely diseased cows. During the first 24 h of the experimental mastitis, the chemotactic response of polymorphonuclear leukocytes decreased in both groups. Recovery of the chemotactic response of white blood cells was more rapid in moderately diseased cows than in severely diseased cows. Possibly, the larger proportion of band neutrophils (the less chemotactically active band neutrophils) partially accounts for the lower chemotactic response of the circulating polymorphonuclear leukocytes during experimental mastitis in the severely diseased cows.

Experimental mastitis was induced by inoculating rear right quarters of 10 healthy cows with 10(3) cfu of Escherichia coli. The chemotactic responses of peripheral blood polymorphonuclear leukocytes at d-6, -5, -2, -1, and immediately prior to inoculation were measured. Chemiluminescence of polymorphonuclear leukocytes was measured immediately prior to inoculation. Severity of the experimental mastitis was assessed by bacterial growth in the inoculated quarters. Results of this study indicated that severity of the experimental mastitis may be predicted by the chemotactic response in vitro of polymorphonuclear leukocytes isolated from the peripheral blood at d 2, d 1, and immediately prior to inoculation. The number of circulating polymorphonuclear leukocytes immediately prior to inoculation also showed a negative relationship with the severity of mastitis. No relationship existed between preinfection chemiluminescence of polymorphonuclear leukocytes and the severity of the experimental mastitis.